Cathepsin B regulates hepatic lipid metabolism by cleaving liver fatty acid-binding protein

α-Tocopherol reduces VLDL secretion through modulation of intracellular ER-to-Golgi transport of VLDL

Avoiding hepatic steatosis is crucial for preventing liver dysfunction, and one mechanism by which this is accomplished is through synchronization of the rate of very low density lipoprotein (VLDL) synthesis with its secretion. Endoplasmic reticulum (ER)-to-Golgi transport of nascent VLDL is the rate-limiting step in its secretion and is mediated by the VLDL transport vesicle (VTV). Recent in vivo studies have indicated that α-tocopherol (α-T) supplementation can reverse steatosis in nonalcoholic fatty liver disease, but its effects on hepatic lipoprotein metabolism are poorly understood. Here, we investigated the impact of α-T on hepatic VLDL synthesis, secretion, and intracellular ER-to-Golgi VLDL trafficking using an in vitro model. Pulse-chase assays using [3H]-oleic acid and 100 µmol/L α-T demonstrated a disruption of early VLDL synthesis, resulting in enhanced apolipoprotein B-100 expression, decreased expression in markers for VTV budding, ER-to-Golgi VLDL transport, and reduced VLDL secretion. Additionally, an in vitro VTV budding assay indicated a significant decrease in VTV production and VTV-Golgi fusion. Confocal imaging of lipid droplet (LD) localization revealed a decrease in overall LD retention, diminished presence of ER-associated LDs, and an increase in Golgi-level LD retention. We conclude that α-T disrupts ER-to-Golgi VLDL transport by modulating the expression of specific proteins and thus reduces VLDL secretion.

Protein-metabolite association studies identify novel proteomic determinants of metabolite levels in human plasma

Although many novel gene-metabolite and gene-protein associations have been identified using high-throughput biochemical profiling, systematic studies that leverage human genetics to illuminate causal relationships between circulating proteins and metabolites are lacking. Here, we performed protein-metabolite association studies in 3,626 plasma samples from three human cohorts. We detected 171,800 significant protein-metabolite pairwise correlations between 1,265 proteins and 365 metabolites, including established relationships in metabolic and signaling pathways such as the protein thyroxine-binding globulin and the metabolite thyroxine, as well as thousands of new findings. In Mendelian randomization (MR) analyses, we identified putative causal protein-to-metabolite associations. We experimentally validated top MR associations in proof-of-concept plasma metabolomics studies in three murine knockout strains of key protein regulators. These analyses identified previously unrecognized associations between bioactive proteins and metabolites in human plasma. We provide publicly available data to be leveraged for studies in human metabolism and disease.

Decreased MARCKS Protein Expression in Kidney Cortex Membrane Fractions of Cathepsin B Knockout Mice Is Associated with Reduced Lysophosphatidylcholine and Protein Kinase C Activity

Cathpesin B is a multi-functional protease that plays numerous roles in physiology and pathophysiology. We hypothesized that actin cytoskeleton proteins that are substrates of cathepsin B, various lipids, and kinases that are regulated by lipids would be down-regulated in the kidney of cathepsin B knockout mice. Here, we show by Western blot and densitometric analysis that the expression and proteolysis of the actin cytoskeleton proteins myristoylated alanine-rich C-kinase substrate (MARCKS) and spectrin are significantly reduced in kidney cortex membrane fractions of cathepsin B knockout mice compared to C57B6 wild-type control mice. Lipidomic results show that specific lipids are increased while other lipids, including lysophosphatidylcholine (LPC) species LPC (16:0), LPC (18:0), LPC (18:1), and LPC (18:2), are significantly decreased in membrane fractions of the kidney cortex from Cathepsin B null mice. Protein Kinase C (PKC) activity is significantly lower in the kidney cortex of cathepsin B knockout mice compared to wild-type mice, while calcium/calmodulin-dependent protein kinase II (CaMKII) activity and phospholipase D (PLD) activity are comparable between the two groups. Together, these results provide the first evidence of altered actin cytoskeleton organization, membrane lipid composition, and PKC activity in the kidneys of mice lacking cathepsin B.

Intracellular Construction of Cathepsin B-Guided Gadolinium Nanoparticles for Enhanced T2 -Weighted MR Tumor Imaging

Magnetic resonance imaging (MRI) is a superior and noninvasive imaging technique with unlimited tissue penetration depth and superb spatiotemporal resolution, however, using intracellular self-assembly of Gd-containing nanoparticles to enhance the T₂ -weighted MR contrast of cancer cells in vivo for precise tumor MRI is rarely reported. The lysosomal cysteine protease cathepsin B (CTSB) is regarded as an attractive biomarker for the early diagnosis of cancers and metastasis. Herein, taking advantage of a biocompatible condensation reaction, a "smart" Gd-based CTSB-responsive small molecular contrast agent VC-Gd-CBT is developed, which can self-assemble into large intracellular Gd-containing nanoparticles by glutathione reduction and CTSB cleavage to enhance the T₂ -weighted MR contrast of CTSB-overexpressing MDA-MB-231 cells at 9.4 T. In vivo T₂ -weighted MRI studies using MDA-MB-231 murine xenografts show that the T₂ -weighted MR contrast change of tumors in VC-Gd-CBT-injected mice is distinctly larger than the mice injected with the commercial agent gadopentetate dimeglumine, or co-injected with CTSB inhibitor and VC-Gd-CBT, indicating that the accumulation of self-assembled Gd-containing nanoparticles at tumor sites effectively enhances the T₂ -weighted MR tumor imaging. Hence, this CTSB-targeted small molecule VC-Gd-CBT has the potential to be employed as a T₂ contrast agent for the clinical diagnosis of cancers at an early stage.

Breast density and estradiol are associated with distinct different expression patterns of metabolic proteins in normal human breast tissue in vivo

Background

Breast density and exposure to sex steroids are major risk factors for breast cancer. The local microenvironment plays an essential role in progression of breast cancer. Metabolic adaption is a major hallmark of cancer. Whether proteins from the extracellular space regulating metabolism are affected in breast cancer, dense breasts or by estrogen exposure are not yet fully elucidated.

Methods

Women with breast cancer, postmenopausal women with normal breast tissue with varying breast density or premenopausal women with breasts exposed to high levels of estradiol were included in the study. Microdialysis was used to collect proteins from the extracellular space in vivo in 73 women; 12 with breast cancer, 42 healthy postmenopausal women with different breast densities, and 19 healthy premenopausal women. Breast density was determined as lean tissue fraction (LTF) using magnetic resonance imaging. Data were evaluated in a murine breast cancer model. We quantified a panel of 92 key proteins regulating metabolism using proximity extension assay.

Results

We report that 29 proteins were upregulated in human breast cancer. In dense breasts 37 proteins were upregulated and 17 of these were similarly regulated as in breast cancer. 32 proteins correlated with LTF. In premenopausal breasts 19 proteins were up-regulated and 9 down-regulated. Of these, 27 correlated to estradiol, a result that was confirmed for most proteins in experimental breast cancer. Only two proteins, pro-cathepsin H and galanin peptide, were similarly regulated in breast cancer, dense- and estrogen exposed breasts.

Conclusions

Metabolic proteins may be targetable for breast cancer prevention. Depending on risk factor, this may, however, require different approaches as breast density and estradiol induce distinct different expression patterns in the breast. Additionally, metabolic proteins from the extracellular space may indeed be further explored as therapeutic targets for breast cancer treatment.

Hepatocyte-like cells differentiated from methylmalonic aciduria cblB type induced pluripotent stem cells: A platform for the evaluation of pharmacochaperoning

Methylmalonic aciduria cblB type (MMA cblB type, MMAB OMIM #251110), caused by a deficiency in the enzyme ATP:cob(I)alamin adenosyltransferase (ATR, E.C_2. 5.1.17), is a severe metabolic disorder with a poor prognosis despite treatment. We recently described the potential therapeutic use of pharmacological chaperones (PCs) after increasing the residual activity of ATR in patient-derived fibroblasts. The present work reports the successful generation of hepatocyte-like cells (HLCs) differentiated from two healthy and two MMAB induced pluripotent stem cell (iPSC) lines, and the use of this platform for testing the effects of PCs. The MMAB cells produced little ATR, showed reduced residual ATR activity, and had higher concentrations of methylmalonic acid compared to healthy HLCs. Differential proteome analysis revealed the two MMAB HCLs to show reproducible differentiation, but this was not so for the healthy HLCs. Interestingly, PC treatment in combination with vitamin B₁₂ increased the amount of ATR available, and subsequently ATR activity, in both MMAB HLCs. More importantly, the treatment significantly reduced the methylmalonic acid content of both. In summary, the HLC model would appear to be an excellent candidate for the pharmacological testing of the described PCs, for analyzing the effects of new drugs, and investigating the repurposing of older drugs, before testing in animal models.

Extralysosomal cathepsin B in central nervous system: Mechanisms and therapeutic implications

Cathepsin B (CatB) is a typical cysteine lysosomal protease involved in a variety of physiologic and pathological processes. It is expressed in most cell types and is primarily localized within subcellular endosomal and lysosomal compartments. Emerging scientific evidence indicates that lysosomal leaked CatB is involved in mitochondrial stress, inflammasome activation, and nuclear senescence, but without the acidic environment. CatB is also secreted as a myokine, which is involved in muscle‐brain cross talk and neuronal dendritic remodeling. Lysosomal‐leaked and cellular‐secreted CatB functions are dependent on its enzymatic activity at a neutral pH. In the present review, we summarize the available experimental evidence that mechanistically links extralysosomal CatB to physiological and pathological functions in central nervous system, and their potential for use in therapeutic approaches.

Behind every smile there's teeth: Cathepsin B's function in health and disease with a kidney view

Cathepsin B (CatB) is a very abundant lysosomal protease with endo- and carboxydipeptidase activities and even ligase features. In this review, we will provide a general characterization of CatB and describe structure, structure-derived properties and location-dependent proteolytic actions. We depict CatB action within lysosome and its important roles in lysosomal biogenesis, lysosomal homeostasis and autophagy rendering this protease a key player in orchestrating lysosomal functions. Lysosomal leakage and subsequent escape of CatB into the cytosol lead to harmful actions, e.g. the role in activating the NLPR3 inflammasome, affecting immune responses and cell death. The second focus of this review addresses CatB functions in the kidney, i.e. the glomerulus, the proximal tubule and collecting duct with strong emphasis of its role in pathology of the respective segment. Finally, observations regarding CatB functions that need to be considered in cell culture will be discussed. In conclusion, CatB a physiologically important molecule may, upon aberrant expression in different cellular context, become a harmful player effectively showing its teeth behind its smile.

The multifaceted role of cathepsins in liver disease

Proteases are the most abundant enzyme gene family in vertebrates and they execute essential functions in all living organisms. Their main role is to hydrolase the peptide bond within proteins, a process also called proteolysis. Contrary to the conventional paradigm, proteases are not only random catalytic devices, but can perform highly selective and targeted cleavage of specific substrates, finely modulating multiple essential cellular processes. Lysosomal protease cathepsins comprise 3 families of proteases that preferentially act within acidic cellular compartments, but they can also be found in other cellular locations. They can operate alone or as part of signalling cascades and regulatory circuits, playing important roles in apoptosis, extracellular matrix remodelling, hepatic stellate cell activation, autophagy and metastasis, contributing to the initiation, development and progression of liver disease. In this review, we comprehensively summarise current knowledge on the role of lysosomal cathepsins in liver disease, with a particular emphasis on liver fibrosis, non-alcoholic fatty liver disease and hepatocellular carcinoma.

Genome-wide scan for common variants associated with intramuscular fat and moisture content in rainbow trout

BACKGROUND

Genetic improvement of fillet quality attributes is a priority of the aquaculture industry. Muscle composition impacts quality attributes such as flavor, appearance, texture, and juiciness. Fat and moisture make up about ~ 80% of the tissue weight. The genetic architecture underlying the fat and moisture content of the muscle is still to be fully explored in fish. A 50 K gene transcribed SNP chip was used for genotyping 789 fish with available phenotypic data for fat and moisture content. Genotyped fish were obtained from two consecutive generations produced in the National Center for Cool and Cold Water Aquaculture (NCCCWA) growth-selective breeding program. Estimates of SNP effects from weighted single-step GBLUP (WssGBLUP) were used to perform genome-wide association (GWA) analysis to identify quantitative trait loci (QTL) associated with the studied traits.

RESULTS

Using genomic sliding windows of 50 adjacent SNPs, 137 and 178 SNPs were identified as associated with fat and moisture content, respectively. Chromosomes 19 and 29 harbored the highest number of SNPs explaining at least 2% of the genetic variation in fat and moisture content. A total of 61 common SNPs on chromosomes 19 and 29 affected the aforementioned traits; this association suggests common mechanisms underlying intramuscular fat and moisture content. Additionally, based on single-marker GWA analyses, 8 and 24 SNPs were identified in association with fat and moisture content, respectively.

CONCLUSION

SNP-harboring genes were primarily involved in lipid metabolism, cytoskeleton remodeling, and protein turnover. This work provides putative SNP markers that could be prioritized and used for genomic selection in breeding programs.

The Ins and Outs of Cathepsins: Physiological Function and Role in Disease Management

Cathepsins are the most abundant lysosomal proteases that are mainly found in acidic endo/lysosomal compartments where they play a vital role in intracellular protein degradation, energy metabolism, and immune responses among a host of other functions. The discovery that cathepsins are secreted and remain functionally active outside of the lysosome has caused a paradigm shift. Contemporary research has unraveled many versatile functions of cathepsins in extralysosomal locations including cytosol and extracellular space. Nevertheless, extracellular cathepsins are majorly upregulated in pathological states and are implicated in a wide range of diseases including cancer and cardiovascular diseases. Taking advantage of the differential expression of the cathepsins during pathological conditions, much research is focused on using cathepsins as diagnostic markers and therapeutic targets. A tailored therapeutic approach using selective cathepsin inhibitors is constantly emerging to be safe and efficient. Moreover, recent development of proteomic-based approaches for the identification of novel physiological substrates offers a major opportunity to understand the mechanism of cathepsin action. In this review, we summarize the available evidence regarding the role of cathepsins in health and disease, discuss their potential as biomarkers of disease progression, and shed light on the potential of extracellular cathepsin inhibitors as safe therapeutic tools.

Cathepsin B inhibition ameliorates the non-alcoholic steatohepatitis through suppressing caspase-1 activation

Non-alcoholic fatty liver disease (NAFLD) has emerged as the most common chronic liver disease. NLRP3 inflammasome activation has been widely studied in the pathogenesis of NAFLD. Cathepsin B (CTSB) is a ubiquitous cysteine cathepsin, and the role of CTSB in the progression and development of NAFLD has received extensive concern. However, the exact roles of CTSB in the NAFLD development and NLRP3 inflammasome activation are yet to be evaluated. In the present study, we used methionine choline-deficient (MCD) diet to establish mice NASH model. CTSB inhibitor (CA-074) was used to suppress the expression of CSTB. Expressions of CTSB and caspase-1 were evaluated by immunohistochemical staining. Serum IL-1β and IL-18 levels were also determined. Palmitic acid was used to stimulate Kupffer cells (KCs), and protein expressions of CTSB, NLRP3, ASC (apoptosis-associated speck-like protein containing CARD), and caspase-1 in KCs were detected. The levels of IL-1β and IL-18 in the supernatant of KCs were evaluated by enzyme-linked immunosorbent assay (ELISA). Our results showed that CTSB inhibition improved the liver function and reduced hepatic inflammation and ballooning, and the levels of pro-inflammatory cytokines IL-1β and IL-18 were decreased. The expressions of CTSB and caspase-1 in liver tissues were increased in the NASH group. In in vitro experiments, PA stimulation could increase the expressions of CTSB and NLRP3 inflammasome in KCs, and CTSB inhibition downregulated the expression of NLRP3 inflammasome in KCs, when challenged by PA. Moreover, CTSB inhibition effectively suppressed the expression and activity of caspase-1 and subsequently secretions of IL-1β and IL-18. Collectively, these results suggest that CTSB inhibition limits NLRP3 inflammasome-dependent NASH formation through regulating the expression and activity of caspase-1, thus providing a novel anti-inflammatory signal pathway for the therapy of NAFLD.

Reticulon 3 regulates very low density lipoprotein secretion by controlling very low density lipoprotein transport vesicle biogenesis

Secretion of very low density lipoprotein (VLDL) by the liver is an important physiological process; however, the rate of VLDL secretion is determined by its transport from the endoplasmic reticulum (ER) to the Golgi. This transport event is facilitated by a specialized ER-derived vesicle, the VLDL transport vesicle (VTV). We have reported earlier a detailed VTV proteome, which revealed that reticulon 3 (RTN3) is uniquely present in the VTV. Our immunoblotting and electron microscopic data demonstrate that RTN3 is enriched in the VTV; however, other ER-derived vesicles do not contain RTN3. Co-immunoprecipitation data coupled with confocal microscopic analyses strongly suggest that RTN3 interacts with VLDL core protein, apoB100, at the ER level. Our data show that either blocking of RTN3 using specific antibodies or RTN3 knockdown resulted in significant reduction in VTV biogenesis from hepatic ER membranes. Additionally, VLDL secretion from hepatocytes was significantly decreased when RTN3 was silenced by RTN3 siRNA. We conclude that RTN3 regulates VLDL secretion by controlling VTV-mediated ER-to-Golgi transport of nascent VLDL.